全文获取类型
收费全文 | 2588篇 |
免费 | 68篇 |
国内免费 | 5篇 |
专业分类
化学 | 1787篇 |
晶体学 | 179篇 |
力学 | 16篇 |
数学 | 337篇 |
物理学 | 342篇 |
出版年
2021年 | 26篇 |
2020年 | 41篇 |
2019年 | 38篇 |
2016年 | 40篇 |
2015年 | 35篇 |
2014年 | 43篇 |
2013年 | 76篇 |
2012年 | 87篇 |
2011年 | 94篇 |
2010年 | 50篇 |
2009年 | 51篇 |
2008年 | 79篇 |
2007年 | 66篇 |
2006年 | 71篇 |
2005年 | 69篇 |
2004年 | 55篇 |
2003年 | 64篇 |
2002年 | 57篇 |
2001年 | 37篇 |
2000年 | 33篇 |
1999年 | 40篇 |
1998年 | 26篇 |
1997年 | 39篇 |
1996年 | 37篇 |
1995年 | 24篇 |
1994年 | 37篇 |
1993年 | 35篇 |
1992年 | 26篇 |
1991年 | 26篇 |
1989年 | 37篇 |
1988年 | 24篇 |
1987年 | 31篇 |
1986年 | 22篇 |
1985年 | 37篇 |
1984年 | 25篇 |
1983年 | 33篇 |
1982年 | 32篇 |
1981年 | 23篇 |
1980年 | 38篇 |
1979年 | 31篇 |
1978年 | 32篇 |
1977年 | 27篇 |
1975年 | 24篇 |
1972年 | 20篇 |
1959年 | 69篇 |
1958年 | 65篇 |
1957年 | 44篇 |
1956年 | 80篇 |
1955年 | 57篇 |
1954年 | 89篇 |
排序方式: 共有2661条查询结果,搜索用时 9 毫秒
101.
Dr. Mahendra K. Sharma Dr. Sebastian Blomeyer Beate Neumann Dr. Hans-Georg Stammler Priv.-Doz. Dr. Rajendra S. Ghadwal 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(35):8249-8253
The first divinyldiarsenes [{(NHC)C(Ph)}As]2 (NHC=IPr 3 a , SIPr 3 b ; IPr=C{(NAr)CH}2; SIPr=C{(NAr)CH2}2; Ar=2,6-iPr2C6H3) are reported. Compounds 3 a and 3 b were prepared by the reduction of corresponding chlorides {(NHC)C(Ph)}AsCl2 (NHC=IPr 2 a , SIPr 2 b ) with Mg. Calculations revealed a small HOMO–LUMO energy gap of 3.86 ( 3 a ) and 4.24 eV ( 3 b ). Treatment of 3 a with (Me2S)AuCl led to the cleavage of the As=As bond to restore 2 a , which is expected to proceed via the diarsane [{(IPr)C(Ph)}AsCl]2 ( 4 ). Remarkably, 4 as well as 2 a can be selectively accessed on treatment of 3 a with an appropriate amount of C2Cl6. Moreover, 3 a readily reacts with PhEEPh (E=Se or Te) at room temperature to give {(IPr)C(Ph)}As(EPh)2 (E=Se 5 a ; Te 5 b ), revealing the cleavage of As=As and E−E bonds and the formation of As−E bonds. Such highly selective stepwise oxidation ( 3 a → 4 → 2 a ) and bond metathesis ( 3 a → 5 a , b ) reactions are unprecedented in main-group chemistry. 相似文献
102.
In this contribution we report on fluorotrimethyl[(Z)-pentafluoropropen-1-yl]phosphorane as a phosphorus based fluorinating reagent. Its solid state structure can be described as a trigonal bipyramid featuring elongated axial bonds due to the formation of a 3-center 4-electron bond. Abstraction of the fluoride ion leads to a shortening of the axial P–C bond. Thus the title compound can be utilized for substitution of bromine with fluorine and for the transfer of fluoride ions onto electrophilic compounds. Reaction with Sn(C2F5)2Br2 afforded salt [P(CH3)3(C3F5)]2[Sn(C2F5)2F4]. When fluorotrimethyl[(Z)-pentafluoropropen-1-yl]phosphorane was treated with P(C2F5)2F the primarily produced anion is sufficiently nucleophilic to attack the propenyl group of the cation in β-position to the phosphorus atom to yield zwitterionic [Me3PCF=C(CF3)–PF3(C2F5)2]. 相似文献
103.
Dominik Schauenburg Dr. Mohammad Divandari Dr. Kevin Neumann Christoph A. Spiegel Thomas Hackett Yi-Chung Dzeng Prof. Dr. Nicholas D. Spencer Prof. Dr. Jeffrey W. Bode 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(34):14764-14771
We report the synthesis of monomers for atom-transfer radical polymerization (ATRP) and a reversible addition-fragmentation chain transfer (RAFT) agent bearing trifluoroborate iminiums (TIMs), which are quantitatively converted into potassium acyltrifluoroborates (KATs) after polymerization. The resulting KAT-containing polymers are suitable for rapid amide-forming ligations for both post-polymerization modification and polymer conjugation. The polymer conjugation occurs rapidly, even under dilute (micromolar) aqueous conditions at ambient temperatures, thereby enabling the synthesis of a variety of linear and star-shaped block copolymers. In addition, we applied post-polymerization modification to the covalent linking of a photocaged cyclic antibiotic (gramicidin S) to the side chains of the KAT-containing copolymer. Cellular assays revealed that the polymer–antibiotic conjugate is biocompatible and provides efficient light-controlled release of the antibiotic on demand. 相似文献
104.
Robin F. Weitkamp Beate Neumann Hans‐Georg Stammler Berthold Hoge 《Angewandte Chemie (International ed. in English)》2020,59(14):5494-5499
We report on the first examples of isolated silanol–silanolate anions, obtained by utilizing weakly coordinating phosphazenium counterions. The silanolate anions were synthesized from the recently published phosphazenium hydroxide hydrate salt with siloxanes. The silanol–silanolate anions are postulated intermediates in the hydroxide‐mediated polymerization of aryl and alkyl siloxanes. The silanolate anions are strong nucleophiles because of the weakly coordinating character of the phosphazenium cation, which is perceptible in their activity in polysiloxane depolymerization. 相似文献
105.
Reflectivity spectra of CuInSe2, CuInTe2 and CuGaTe2 are measured in the photon energy range from 2 to 9 eV. All compounds exhibit nearly the same reflectivity structures, and a comparison with measurements on the other Cu-III-VI2 semiconductors shows that the band structures of all Cu-III-VI2 compounds should be very similar except the energy range near the fundamental edge. 相似文献
106.
V. V. Mesilov V. R. Galakhov B. A. Gizhevskii A. S. Semenova D. G. Kellerman M. Raekers M. Neumann 《Physics of the Solid State》2013,55(5):943-948
The charge states of ions in nanostructured lithium cobaltite prepared by severe plastic deformation under pressure have been determined using X-ray absorption spectroscopy and photoelectron spectroscopy, as well as calculations of the atomic multiplets with allowance for the charge transfer. It has been found that small deformations (pressures up to 5 GPa and angles of anvil rotation up to 30°) lead to the generation of lithium vacancies in the bulk of the nanostructured material and the formation of the Li2O phase on the surface. The charge compensation occurs at the expense of holes in oxygen 2p states; the electronic configuration of cobalt ions is 3d 6 L, where L is a hole in oxygen 2p states. It has been shown that nanostructured lithium cobaltite belongs to the class of insulators with a negative charge transfer energy. An increase in the degree of deformation of lithium cobaltite (at a pressure up to 8 GPa) leads to the formation of Co2+ ions (with the electronic configuration 3d 7). 相似文献
107.
Beate Bussemer Klaus-Peter Schrder Joachim Sauer 《Solid state nuclear magnetic resonance》1997,9(2-4)
A recent shell-model potential parameterized on ab initio data is used for predicting the all-silica structures of zeolites MFI, MEI, MTW, TON, FAU and of α-quartz. Cluster models are defined around each site and the 29Si NMR shielding constants are calculated by ab initio techniques (GIAO-HF). Good agreement with observed 29Si NMR chemical shifts is found. Comparison is made with shifts calculated for observed structures. The structures predicted by the ab initio shell-model potential prove as accurate as the observed ones when judged on the quality of the calculated 29Si NMR spectra. 相似文献
108.
K.-U. Neumann F. Kusmartsev H.-J. Lauter O. Schärpf T.J. Smith K.R.A. Ziebeck 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):5-9
A polarized neutron scattering investigation of the flux line lattice in the type-II superconductor niobium is reported. A
modulation of the nuclear lattice has been detected, and the magnitude of the first Fourier component of the lattice distortion
established relative to the magnitude of the magnetic scattering. This constitutes the first experimental observation of lattice
distortions due to the presence of magnetic flux lines within the bulk of a type-II superconductor. Using a simple microscopic
model the lattice distortion in niobium is estimated. A new mechanism is suggested for the coupling of the flux line lattice
to the crystallographic lattice. The experimental technique opens up the possibility of investigating the microscopic mechanism
of flux line - nuclear lattice interactions, in particular the pinning of flux lines within the bulk of a type-II superconductor.
Received: 8 August 1997 / Accepted: 16 October 1997 相似文献
109.
A model system consisting of one strongly polar molecule dissolved in a pool of 499 moderately polar particles is studied in a molecular dynamics experiment. A pair potential of Stockmayer type is used. The structure of the solution has been analysed in terms of g(R), f(R), h Δ(R), h D(R), g(R, x), and Voronoi polyhedra. Solvent-solvent spatial correlations are presented and compared to solute-solvent properties already discussed in part I. Voronoi polyhedra analysis is also used to define structural relaxation around solute or solvent particles unambiguously. The dynamics of the system is described by means of angular momentum, orientational, and velocity autocorrelation functions. The solute's influence on the dynamics of the solvent is analysed by a polyhedral decomposition of time correlation functions. 相似文献
110.
In the theory of iterative methods, the classical Stein-Rosenberg theorem can be viewed as giving the simultaneous convergence (or divergence) of the extrapolated Jacobi (JOR) matrix Jω and the successive overrelaxation (SOR) matrix , in the case when the Jacobi matrix J1 is nonnegative. As has been established recently by Buoni and Varga, necessary and sufficient conditions for the simultaneous convergence (or divergence) of Jω and have been established which do not depend on the assumption that J1 is nonnegative. Our aim here is to extend these results to the singular case, using the notion of semiconvergence. In particular, for a real singular matrix A with nonpositive off-diagonal entries, we find conditions (Theorem 3.4) which imply that Jω and simultaneously semiconverge for all ω in the real interval [0,1). 相似文献